There are a number of reasons why you shouldn't cast Michael Fassbender in your movie and have him wear a giant, papier-mâché head throughout, but most central is that he has a wonderfully emotive face (stern in Shame, wild-eyed in 12 Years a Slave, creepy in Prometheus), one that's central to his acting abilities and that happens to be as attractive as it is talented. Put the two together, and the mask stands to get you accused of both thespian and aesthetic crimes.

Yet covering up Fassbender is exactly what Lenny Abrahamson improbably pulls off in his dark comedy Frank, a film about a mid-30s rock star based only loosely but obviously on Chris Sievey, an artist who performed in the '80s and '90s under the name Frank Sidebottom while wearing a fake head, and who one of the film's writers, Jon Ronson, toured with at the time. That Fassbender plays the role of the disguised rocker fits well with the movie's tone: putting one of the biggest actors in Hollywood behind a mask lends a further rebellious streak to an already sardonic film.

The film's protagonist and narrator, however, is not Frank but Jon (Domhnall Gleeson), an office drone with dreams of rock stardom. We meet him in the film's first scene as he sits by the beach of his hometown, writing awful songs in his head based on the actions of passersby. ("Ladies with babies. That's how it works!" is one of his masterpieces.)

By chance, Jon happens upon Frank's band, The Soronprfbs, as pianist Lucas is being forcibly restrained from drowning himself in the ocean. With a gig that night, the band needs a new member, and Jon steps in, eventually getting invited to a remote lakeside cabin where the band plans to record an album.

To start, Frank is largely kept in the background as the enigmatic leader of the mysterious group. When he emerges, though, what stands out is how decidedly uncool he is. While performing, he's a post-rock Jim Morrison, producing moody spoken word over the band's noise-rock sounds. But otherwise, Frank is sweet-natured, telling everyone which positive facial expressions he's making behind his mask and reacting all too enthusiastically to any band member's new idea.

The rest of the band, particularly the scowling Clara (Maggie Gyllenhaal), are not nearly as peppy, but they're equally unfit for the limelight. Filtered through Jon's enthusiasm, the band may seem like geniuses living in artistic isolation waiting for inspiration to strike. But a closer look reveals they're in fact a lonesome bunch finding a simple satisfaction in their music.

The trouble is that Jon begins to tweet, blog and post YouTube videos of their recording process, turning the band into a mythic presence online. Much of the film turns on this tension between the grandiose image that the overzealous Jon — who uses his inheritance to pay for rent at the cabin — has of the band, the dreams of grandeur he inspires in Frank, and the reality check that necessarily awaits both of them in the future.

That conflict mirrors a more structural tension in the film as it careens between rich, dark comedy and insensitivity, particularly in its treatment of mental illness. From the start, it's explicitly acknowledged that Frank and the band's manager, Don (Scoot McNairy), were hospitalized earlier in their lives. The film, though, plays their ailments and their impulsive behavior for laughs. Don, it turns out, has a mannequin fetish, and a scene where he tries to drown himself in a lake is one of the more comically surreal moments in the movie.

For most of Frank, Abrahamson walks this potentially offensive line largely by playing it as part of Jon's ignorance: At the cabin, Jon excitedly explains, "I have found my abusive childhood, my mental hospital, that which pushes me to my furthest corners." The film quite clearly means to deride the notion — heard particularly when discussing outsider art or musicians like Daniel Johnston, who Abrahamson has said also inspired the film — that mental illness can somehow be a path to boosted creativity.

In this attempt to push the limits of its comedy, it helps that the film is consistently funny (a scene where Frank plays the band his "most likeable song ever" is one of the year's best scenes) and that Abrahamson is clearly an equal-opportunity offender. He mocks Jon, the art-school pretension of The Soronprfbs, the absurdities of their 18-month recording retreat, and audiences' fixation on reclusive artists with the same uninhibited energy.

Yet toward the end, there are suggestions of a lack of compassion for some of these characters. After letting the audience think it's OK, if not necessarily tasteful, to laugh at jokes about attempted suicide, the story shifts and we are left to question if there was ever any humor in it at all. Whether the film fully accomplishes this transition from comical (and almost sneering) to empathetic is its own open question. My vote says it does, but it's a fine line, one that Frank necessarily risks overstepping with its complex, but worthwhile, attempt at being facetious, satirical and emotionally bare all at once.

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